The invention relates to a measuring apparatus having at least one measuring unit, which in turn is provided with at least one optical emitter and at least one optical receiver, and at least one light source, which is connected with the measuring unit via a light guide, as well as a detector that is connected with the measuring unit via a light guide.
Known color-measuring systems have a single fixed emitter and receiver arrangement. In this connection, conventionally only one light source and only one detector are used. The illumination is frequently effected with halogen light sources or xenon light sources in a pulse or continuous mode operation. The light source illuminates the sample to be measured directly, although it is generally coupled to a light guide. The light guide then illuminates the probe either directly via a round fiber bundle, an annular light arrangement of fibers, or a plurality of fiber bundles that illuminate the sample to be measured from different directions. It is furthermore known to columnate or parallelize the light exiting the light guide via an optical emitter in order to achieve a defined direction of illumination. As detectors, often either three photocells having appropriate optical filters are utilized pursuant to the three-region method, or a three-element photocell having vapor-coated optical filters is used. It is furthermore known to use spectral photometers having a rotary grid, or to use diode line-scanning or CCD line-scanning spectrometers having a fixed grid. The coupling of the detector is generally effected via a light guide. In this connection, an optical measuring head for parallelization, that is also designated as a receiving measuring head, could be disposed between the sample to be measured and the light guide. With the known arrangements, only one fixed illumination and receiving geometry is utilized for the color measurement at the sample.
Furthermore known are measuring apparatus for color-measuring systems that, at a number of angles, measure characteristics, in particular the color of an object that is to be measured. With one type of the known measuring systems, the measurement is effected either with a fixed illumination angle and a plurality of fixed receiving angles, or with a fixed receiving angle and a plurality of fixed illumination angles. Measuring systems of this type of construction are particularly unsuitable for use when measuring the angle-dependent color characteristics of novel lacquers and colors. For an adequate characterization of the colors, neither a single illumination angle nor a single receiving angle can be fixedly prescribed. This generally means that a plurality of detector units, such as, for example, spectrometers, and possibly also a plurality of illumination units, must be utilized, which leads to higher costs due to the increased expenditure for components. Furthermore, all of the measuring geometries must be simultaneously disposed over the sample, as a result of which the geometric possibilities for the arrangement of the measuring heads is limited, and not all desired illumination angles and receiving angles can be measured.
With a second known type of measuring system, the illumination angle and the receiving angle can be adjusted manually or in a motorized manner. Such a goniometer offers the possibility of setting any desired illumination angle by two movable measuring heads.
However, goniometers are difficult to adjust and, due to the susceptibility to adjustment, they can be used only or laboratory purposes, but not for routine production controls. Furthermore, the expenditure for apparatus for automated goniometers having motor control is high, as a result of which the apparatus are expensive. Furthermore, generally light sources and detectors are coupled to the measuring heads via light guides, so that a movement of the measuring heads leads to an alteration in the bending curves of the light guides and changes in the spectral transmission of the optical fibers occurs. Therefore, imprecise measurements result with goniometers due to a movement of the measuring heads relative to the light source and the detector, which have a disadvantageous effect upon the precision of the color value determination.
Due to the increasing use of lacquers and colors having angle-dependent color characteristics, i.e. a color characteristic that is a function of the illumination angle as well as of the viewing or receiving angle, the demand for precise multiple-angle color measuring apparatus is increasing.
It is therefore an object of the invention to provide a measuring apparatus with which the above-mentioned drawbacks of conventional measuring systems are avoided, and in particular to enable a determination that is as precise as possible of the color characteristics as a function of the angle and to accomplish this at different illumination and receiving geometries.